The Pentagon halted its cooperation with Marvel Studios' blockbuster movie The Avengers because the Defense Department didn't think a movie about superheroes, Norse Gods and intergalactic invasions was sufficiently realistic in its treatment of military bureaucracy.

Moviegoers and comic fans know that S.H.I.E.L.D., led by Samuel L. Jackson's super-spy Nick Fury, is an international peacekeeping/global surveillance/crisis response/quasi-military organization. But its relationship with the United States is murky. And that basically stopped the U.S. military, which is normally eager to cooperate with the film industry on blockbuster movies, from teaming up with the Avengers.

"We couldn't reconcile the unreality of this international organization and our place in it," Phil Strub, the Defense Department's Hollywood liaison, tells Danger Room. "To whom did S.H.I.E.L.D. answer? Did we work for S.H.I.E.L.D.? We hit that roadblock and decided we couldn't do anything" with the film.

Well, almost anything. In the movie's climactic Manhattan fight scene, New York National Guardsmen show up to try to help police and firemen contain - spoiler alert - the damage wrought by a cosmic invasion. The Defense Department allowed Marvel to film Humvees for the scene.

But attentive moviegoers may have noticed the U.S. military's latest stealth jets, the F-22 Raptors and what looked like F-35 Joint Strike Fighters, aboard S.H.I.E.L.D.'s airborne helicarrier, an awe-inspiring, tilt-rotor aircraft carrier. (One assumes that thing has the runway space necessary for a Raptor take-off - and S.H.I.E.L.D. super-scientists fixed the jet's oxygen woes.) The fighters were "digitally inserted" by the studio, Strub explains, not actual planes provided by the U.S. military.

But the ambiguity around what exactly S.H.I.E.L.D. is provides a vexing complication. If it's an American governmental agency, what kind of constitutional authority does it exercise over the military? If it's an international body, as the movie text suggests and Strub determined, are U.S. military personnel and equipment on loan to it through some kind of United Nations Security Council resolution? The questions may seem picayune, but they're precisely the stuff that can cause an image-conscious military to yank its cooperation from a movie.

The comics have fudged the issue for decades. Marvel now describes it as an "extra-government" body, although many takes on the organization have clearly emphasized its international character. Yet U.S. presidents have fired S.H.I.E.L.D. directors (Fury, Tony Stark/Iron Man) and appointed others (Norman Osborn/The Green Goblin, incumbent Steve Rogers/Captain America) - although that might operate by an informal international understanding, much like the U.S. appoints the director of the World Bank.

Either way, the ambiguity prevented the Avengers from assembling beside the U.S. military. "It just got to the point where it didn't make any sense," Strub laments. And now comic nerds have another data point to bring up during continuity debates about what exactly S.H.I.E.L.D. is.